The interaction of aqueous solutions of calcium and phosphate ions with apatitic substrates is being studied. Parameters being investigated are the rate and extent of the interchange of ionic calcium and phosphate between solution and substrate, and the effects this interchange has on substrate morphology and structure. Particular attention is being directed to the mechanisms by which new material is accreted onto the apatitic substrate under conditions of solution supersaturation. The effect on apatite crystals of rapid, cyclical alterations in solution conditions such as can be affected by sudden pH changes is also being studied, as is the role of inorganic and biorganic adducts in modifying the above parameters. The purpose of these studies is to establish the fundamental physicochemical mechanisms by which enamel apatite responds to changes in oral fluid environment.